1. Field of the Invention
The present invention relates to a resin coated carrier for electrophotography and to a two-component developer for electrophotography made therefrom.
2. Related Art
Two-component developers used in an electrophotographic system are composed of a toner and a carrier. The carrier has a function which yields triboelectric charge and gives desired charge to the toner, by mixing and stirring with the toner in a developing tank and which forms a toner image by transferring the charged toner to an electrostatic latent image on a photoconductor. Then, the carrier is repeatedly used by returning from the surface of a magnet roll to the inside of the developing tank and by mixing and stirring with newly supplied toner again.
Therefore, the carrier is required to provide the toner with properties which are always desired under all environments for the entire using period, and in particular, to provide it with stable charging properties.
However, conventional two-component developers have spent phenomenon in which toner adheres on the surface of carrier by stress such as collision of carriers in stirring, friction between a developing tank and carriers, or the like. In order to prevent the carrier from deteriorating by the spent phenomenon, various resins have ever been applied on the surface of core particles. When the surface of the core particle is covered with resin, since the surface of the carrier is made to be smooth, it is difficult to adhere the toner to the carrier, and it is also difficult for the spent phenomenon to occur. Therefore, the life of the developer can be extended. Furthermore, charging characteristics and electric resistance of the carrier, etc., can be controlled by selecting the resin.
In addition, since the core particle does not directly receive influence of the environment by being covered with the resin, it is difficult to change characteristics of the carrier to environmental conditions such as temperature change, humidity change, etc.
Thus, practical use is drastically improved when the surface of the core particle of the carrier is covered with the resin.
As a resin which covers the surface of the core particle, silicone resins, silicone grafted resins, acrylic resins, styrene resins, urethane resins, epoxy resins, polyamide resins, polyester resins, acetal resins, polycarbonate resins, phenol resins, vinyl resins (vinyl chloride resin, polyvinyl acetate, etc.), cellulose resins, polyolefin resins, styrene-acrylic copolymer resins, styrene-butadiene copolymer resins, rosin resins, fluorided acrylic resins, fluororesins, etc., can be used. The resins have been used alone, as mixed resins, or as copolymer resins of a monomer component thereof.
However, in resin coated carriers produced by the above conventional techniques, peeling or falling out of coated resin film, etc., occurs on the surface of the carrier by stress described above, and characteristics of the carrier such as electrostatic charging amount, carrier resistance, etc., are deteriorated by these phenomena, and there are problems that degradation of image (changing of image density, generation of background fogging in image, etc.) and dusting of toner are generated. In particular, in the case in which the carrier is used under severe conditions such as the case in which strong stirring is required in order to quickly increase electrostatic charge, or the case in which long life is required, the problem that the coated resin film peeling is remarkable. When film strength and abrasion resistance of the coated resin are not sufficient, since the film is inferior in mechanical shock, the film is worn and broken by collision between particles and collision with developing members in the passage of time, and the coated resin peeled from the surface of the carrier is mixed and accumulated in developer. When the coated resin film peels, electric resistance on the surface of the carrier partially changes and sufficient triboelectric charge is not obtained on the toner, and as a result, image concentration is decreased and fogging is generated in the image background. Furthermore, since the peeled coated resin often has high polarity, there are problems that it adheres to toner and to a developing unit, that it adheres to the surface of a photo conductor and background contamination (fogging) and uneven streaks are generated on the image, and that triboelectric charging characteristics of the developer is changed and quality of the image is deteriorated, etc. In a word, in the resin coat carriers made by conventional techniques, there are problem that electrophotographic characteristics are easily affected by peeling of the resin film generated by stress.
In addition, electrostatic charge maintaining ability after leaving is also important, because the developer is left after charging and it is used again. When the electrostatic charge maintaining ability is too low, electrostatic charge is decreased during leaving, and fogging is generated in resuming printing.
As a representative resin used as the coated resin, acrylic resins and silicone resins are mentioned.
The acrylic resin has high adhesive strength to the carrier core material and peeling is unlikely to occur. The acrylic resin coated carrier is superior in electrostatic charge giving ability to the toner, in particular, electrostatic charge giving ability to negative charge toner, and it is used widely; however, there are disadvantages in which anti-spent property and electrostatic charge maintaining ability are inferior.
In addition, the silicone resin coated carrier has low surface energy, friction coefficient is also low, and anti-spent property is superior; however, there is disadvantage in which electrostatic charge giving ability is inferior. That is, the silicone resin coated carrier is inferior in electrostatic charge increasing ability, and fogging is easily generated when the toner is supplied. However, the fogging after leaving is difficult to generate since the electrostatic charge maintaining ability after leaving is superior.
As a resin coated carrier having both superior characteristics of these acrylic resin coated carrier and silicone resin coated carrier, acrylic modified silicone resin coated carriers and silicone modified acrylic resin coated carriers are proposed (Japanese Unexamined Patent Application Publications Nos. H3-31860, H8-234501, and 2001-92190).
However, in the case in which the problem of peeling of the resin film is included, it is not easy to resolve the above problems well, even if the resins for coating are selected.
Additionally, as a core material in the resin coated carrier, ferrite or magnetite is mainly used (Japanese Unexamined Patent Application Publication No. 2001-154416).
The magnetite has generally high saturation magnetization, and it is advantageous for adhering carrier to a photoconductor; however, the stress is large in developing and degradation of the developer is rapid, and there is a problem that durability of the developer is inferior. In addition, it is difficult to obtain a high definition image, since the saturation magnetization is generally high.
Furthermore, the carrier is used as a two-component developer by combining with the toner. Therefore, in order to demonstrate superior characteristics as a two-component developer, characteristics of the toner are also important, and it is desirable that superior triboelectric charging ability, etc., as a developer be had by suitably combining with the toner which conform to characteristics of the carrier.